Anti-skid brake control systems are known in which signals are derived from wheel speed transducers which are representative of deceleration of the vehicle, acceleration of the vehicle, or the wheels, respectively, and which then can determine slippage between the wheels and a road surface. Depending on limiting or predetermined values of such control signals, the brakes of the respective wheels are so controlled that, if blocking of a wheel, or tendency to block is sensed, the wheels are braked only to that limit which is just above the skidding or blocking condition.
The situation may arise that vehicles operate on road surfaces which have highly asymmetrical frictional characteristics. For example, the wheels on one side of the vehicle may be on a dry or rough surface, whereas the wheels of the vehicle on the other side thereof may be on a highly slippery surface, such as on a sheet of ice, snow cover, or wet surface. Upon application of brakes, which will be effective on the side of the vehicle which has the gripping road surface, substantial yawing torques may arise; such yawing torques extend about a vertical axis through the vehicle. Due to braking, the wheels which are on the rough or gripping side of the road will be highly decelerated, thus decelerating the vehicle; the wheels which are on the slippery side of the road will not decelerate much, and thus rotary or yawing torques will have a tendency to turn the vehicle about its vertical axis. This condition arises particularly in trucks, cross-country transport vehicles, and the like.
It has previously been proposed to counteract the high yawing torques by controlling the brakes on the side of the vehicle which has the high frictional engagement with the road in dependence on braking at the side having the low frictional values. It has been proposed to leave the braking pressure on the vehicle side with the high frictional value and not drop it, upon sensing of slippage at the low friction side; the braking pressure is not increased, however, either until the braking pressure applied to the side with the low frictional value is also increased. In this type of operation, pressure at the high-friction side, capable of providing high braking effort and having a high braking effect, is increased only in a specific relationship with respect to the braking effort or braking effect obtainable at the vehicle side operating on a slippery surface.
A vehicle brake anti-block control system of this type is described, for example, in the referenced U.S. Pat. No. 4,288,127, assigned to the assignee of the present application.
The situation may arise under extreme conditions of vehicle operation that the braking distance becomes too long, particularly if the difference in braking effect which can be obtained at the two vehicle sides is high or extreme; this braking effect can also be simulated as far as the wheel speed transducers are concerned by differential friction within the wheel bearings, or by poorly releasing brakes at specific wheels. The overall braking distance, thus, may become excessive since the interaction of build-up of braking pressure at the vehicle side which operates on a high-friction surface is delayed to long. To improve the situation, and match more conditions, thus, the braking pressure P.sub.L of the wheel operating at a higher frictional level is increased by a special control signal E.sub.Z if a predetermined minimum time t.sub.o is exceeded between dropping of braking pressure and subsequent increase of braking pressure at the wheel operating at the slippery surface. The referenced U.S. application Ser. No. 469,741, JONNER, KORASIAK & LEIBER, filed Feb. 26, 1983, now U.S. Pat. No. 4,489,382 describes such an arrangement. This arrangement has the advantage that an optimum compromise between low yawing torques and short braking distance can be obtained. The supervision of the braking pressure is particularly simple since increase of braking pressure at the vehicle side operating at the high-friction surface can be easily controlled by merely monitoring the temporal occurrence of the control signals for the braking pressure inlet and outlet valves of the brakes of the vehicle operating at the slippery side of the roadway.
The braking pressure control, as described, can be matched to various types of operating situations by, for example, relating the application of braking pressure to the brakes at the side with the high-friction surface to vehicle speed.
Limiting yawing torques is particularly important in vehicles with large rolling radii, since the yawing torques should be primarily effective upon braking by asymmetrical frictional conditions between wheel and road surface. Limitation of yawing torques is also necessary for vehicles which should be braked with good vehicle operating control at high speed if the gripping conditions between tires and road surface are different between right and left side of the vehicle.
Differential braking forces occur not only when the wheels of the vehicle operate on road surface portions which have different slipperiness or frictional characteristics at the different sides of the vehicle. They also occur upon braking when the vehicle passes through a curve, or, otherwise, when there is high acceleration transverse to the longitudinal axis of the vehicle. It would be disadvantageous to prevent application of optimum braking effort or braking effects to the wheels of the vehicle by yawing torque limitation, if braking is to be carried out for example in a curve. For deceleration, at least the front wheels of the vehicle passing through a curve should be fully effective, so that the vehicle will be decelerated from excessive speed, or from a limiting speed range.